The role of thyroid supplementation
Still, some obese horses with poor coats and lethargic attitudes have low T3 and T4 concentrations and actually seem to do
better on thyroid hormone supplementation. How does science account for these animals?
Frank and other researchers acknowledge that these overweight, likely insulin-resistant horses that possibly have metabolic
syndrome and those with Cushing's disease and other conditions can show decreased thyroid hormone concentrations. But these
animals are adding to the myth of hypothyroidism in the horse. According to Frank's research, "We have never identified a
horse with persistently low thyroid hormone concentrations—the same horse will have a normal concentration the next time it
is tested." Both Frank and Breuhaus agree that many horses with significant endocrine disorders will have normal thyroid hormone
Perhaps the most persuasive piece of scientific evidence showing that hypothyroidism may not even exist as a clinical entity
in horses is Frank's work with horses that have undergone a thyroidectomy. These horses have had their thyroid glands surgically
removed, so they have no thyroid hormones in their systems. Frank's research showed that even profound hypothyroidism caused
by the absence of thyroid glands did not induce obesity, the regional deposition of fat (crest neck or rump), laminitis or
other signs typically associated with equine hypothyroidism.
Thyroid hormone supplementation has been commonly used in equine veterinary medicine. It has been advocated for anhidrotic
(non-sweating) horses and as a means of increasing fertility in broodmares and stallions. In many cases, both veterinarians
and owners would agree that positive test results are seen with the addition of thyroid hormones. Breuhaus designed a study
to test the thyroid function in anhidrotic horses, and she concluded that "resting concentrations of thyroid hormone and thyroid
stimulating hormone were not different between anhidrotic and control (normal sweating) horses."
In a study, Travis Meredith, DVM, Dipl. ACT, and Ina Dobrinski, Dr.med.vet., MVSc, PhD, Dipl. ACT, looked at 79 Thoroughbred
and Standardbred broodmares between the ages of 2 and 22 years of age. T3 and T4 concentrations were measured initially and
then four hours after administration of TSH. These mares were bred and pregnancy rates were evaluated.
Overall, 50 of 79 mares had low thyroid hormone concentrations. All mares had elevated T3 concentrations after TSH, and T4
concentrations increased after TSH in all horses except for two mares. The 79 mares had a pregnancy rate of 76 percent, which
approaches that for all normal mares, and baseline and stimulated T3 and T4 concentrations were not significantly different
between mares that were pregnant and those that were "open."
"These results suggest that decreased thyroid function is not a common cause of infertility," Meredith and Dobrinski say.
"The practice of indiscriminately treating broodmares with thyroid hormone to enhance fertility appears questionable at this
Frank feels that thyroid hormone supplementation does have some pharmacologic effect on horses, and his research shows that
levothyroxine induces weight loss and increases insulin sensitivity. It may be these actions that slim down broodmares and
make them healthier and, therefore, more fertile. In fact, broodmares that underwent a thyroidectomy continued to cycle despite
the absence of as thyroid gland. These mares had abnormal cycle length but still had follicular activity and ovulated without
These studies seem to indicate that it is highly unlikely that true hypothyroidism exists in horses despite many past years
of inaccurate diagnosis and inappropriate treatment. Continued research and advanced diagnostics may further clarify this
debate, but for now equine practitioners may have to change their mindsets and cease to see hypothyroidism as a disease requiring
Dr. Marcella is an equine practitioner in Canton, Ga.